Stairs often present a formidable physical and psychological obstacle to elders, who are intuitively aware that falls from stairs account for the highest proportion of fall related injuries when compared to all other fall causations. In particular, stair descent is the most common dangerous locomotor activity and accidents during descent outnumber those during ascent by over 3 to 1. Despite these facts, there are little data available on this topic. The investigators in this proposal, therefore, propose a study which after assembling a suitable experimental setting will attempt to define the presently unknown biomechanical characteristics of stair descent. Essentially, three major groups of subjects will be studied. A healthy group of individuals, both young and elderly (70-79 years of age), an elderly group with a single prominent disability, and an elderly group with multiple deficits. Their specific aims are 1) to construct an experimental staircase that will enable the investigation of important biomechanical and visual aspects of stair use to be measured in a valid, safe and effective manner, 2) to define the biomechanical characteristics of stair descent under optimal, extrinsic conditions in healthy, unimpaired young and elderly subjects, 3) to define the biomechanical characteristics of stair descent under non-optimal, extrinsic and visual conditions in healthy, unimpaired young and elderly subjects, and 4) to understand how motor (osteoarthritic patients), somatosensory (neuropathic diabetic model), and multiple impairments (frail elderly) together with degraded extrinsic conditions affect the safety of elders during the different phases of stair descent.